Support hierarchical label in classification task

[cih9088]

Summary

• Ticket: CVS-108587

Exploited existing LabelSchema feature, LabelTree for hierarchical label support. A task can use it through self._task_environment.label_schema.label_tree

How to test

Checklist

• I have added unit tests to cover my changes.​
• I have added integration tests to cover my changes.​
• I have added e2e tests for validation.
• I have added the description of my changes into CHANGELOG in my target branch (e.g., CHANGELOG in develop).​
• I have updated the documentation in my target branch accordingly (e.g., documentation in develop).
• I have linked related issues.

License

• I submit my code changes under the same Apache License that covers the project.
  Feel free to contact the maintainers if that's a concern.
• I have updated the license header for each file (see an example below).

# Copyright (C) 2023 Intel Corporation
# SPDX-License-Identifier: Apache-2.0

[sungmanc]

LGTM, @sovrasov , could you also take a look this ?

[sovrasov]

From my side, I'd expect at least unit tests for that functionality, but it's already merged.

[cih9088]

From my side, I'd expect at least unit tests for that functionality, but it's already merged.

PR for unit tests: #2152